Don't treat `body` as a keyword.

Just recently a user wanted to use `body` as a member in his HTML document class:
http://forum.dlang.org/thread/vbfrootcxaecjgciwlvb@forum.dlang.org
I believe I've seen similar discussions several times in the forums.

Someone suggested to only treat `body` as a keyword where it makes sense. I tried implementing that, and, while not exactly beautiful, it turned out to be surprisingly non-invasive. It's not a keyword at all anymore, but a special identifier.

This same technique is also used for e.g. `scope(success)` among others, and could potentially be applied to other keywords too, but I guess `body` is the one with the greatest potential of conflict, because there is no nice alternative word (apart from capitalization and appending underscores etc.), and it's not a keyword in C/C++, so is freely used in some libraries.